Polysaccharides with complement fixing and macrophage stimulation activity from Opilia celtidifolia, isolation and partial characterisation

AIM OF THE STUDY

The present study is aimed to determine the bioactivity and structure of polysaccharides present in the leaves from the Malian medicinal plant Opilia celtidifolia [Guill. & Perr. Endl. ex Walp (Opiliaceae)].

MATERIALS AND METHODS

The polysaccharides from the leaves of Opilia celtidifolia were isolated from water extracts of the leaves using gelfiltration and anion exchange chromatography giving the fractions Oc50A1 and Oc50A2. Monosaccharide composition was determined by gas chromatography of the derived TMS-derivatives of the methyl-glycosides. Linkages were determined of the partly methylated, partly acetylated alditol acetates obtained after a process including reduction, methylation, hydrolysis, reduction and acetylation followed by GC-MS. Effects on the complement system and the macrophages were determined using specific methods aimed for studying those activities.

RESULTS

The polysaccharide fractions isolated from the leaves of Opilia celtidifolia has high complement fixing activity and induce nitrite oxide release from macrophages in a dose dependent manner. The fractions had an ICH50 of 0.5 and 0.9 microg/ml respectively in the complement fixing assay. They induced the release of 7.2 and 7.3 microM of nitrite oxide from macrophages respectively at a dose of 100 microg/ml. The monosaccharide composition in Oc50A1 and Oc50A2, analysed, showed the presence of arabinose (26.7 and 13.2%), galactose (31.5 and 28%) and galacturonic acid (5.3 and 7.8%) respectively. The Yariv test confirmed the presence of arabinogalactan type II in both fractions. Structural analyses did also show the presence of terminal and 1-4 linked galacturonic acid and terminal and 1-2 linked rhamnose. Endo-polygalacturonanase treatment was performed to isolate the heavily substituted parts of the polysaccharides. These parts contained the same monosaccharides in similar proportion, and showed stronger dose dependent complement fixing activity and also stimulated macrophages to release nitrite oxide.

CONCLUSIONS

The leaves of Opilia celtidifola contains polysaccharides of pectic type that have both complement fixing and macrophage stimulating activity.

Strain-dependent expression of four structurally related rat Ly49 receptors; correlation with NK gene complex haplotype and NK alloreactivity

Natural killer (NK) cells from certain rat strains promptly kill MHC allogeneic lymphocytes in vivo, a rejection phenomenon termed allogeneic lymphocyte cytotoxicity (ALC). ALC can be reproduced in vitro, and is preferentially mediated by a subset of NK cells expressing the Ly49 stimulatory receptor 3 (Ly49s3) in PVG strain rats. Functional studies have suggested that Ly49s3 triggers NK cell alloreactivity, but its importance relative to other Ly49 receptors has not been investigated. In this study, we have characterized three rat Ly49 receptors with close sequence similarity to Ly49s3 in the extracellular region, i.e., Ly49s4, Ly49 inhibitory receptor 3 (Ly49i3), and Ly49i4. Similar to Ly49s3, Ly49s4 mediated cellular activation while Ly49i4 inhibited NK cytolytic function. Ly49s4, -i3, and -i4 all reacted with a previously described anti-Ly49s3 monoclonal antibody (mAb) (DAR13), but not a novel mAb (STOK6), which was shown to be specific for Ly49s3. Expression of these Ly49 receptors varied markedly between inbred strains, in patterns related to their NK gene complex (NKC) haplotype, and ability to mediate ALC. Three major groups of NKC haplotypes could be discerned by restriction fragment length polymorphism analysis. Ly49s3 was present in strains from one of the groups, which corresponded with the "high" ALC responders. Ly49s3 surface expression was also markedly reduced in the presence of its putative MHC class Ib ligand(s) in MHC congenic strains. These data support the notion that Ly49s3 functions as a triggering MHC receptor both in vitro and in vivo. MHC ligands for the other three Ly49 receptors remain to be determined.

The Novel Inhibitory NKR-P1C Receptor and Ly49s3 Identify Two Complementary, Functionally Distinct NK Cell Subsets in Rats

The proximal region of the NK gene complex encodes the NKR-P1 family of killer cell lectin-like receptors which in mice bind members of the genetically linked C-type lectin-related family, while the distal region encodes Ly49 receptors for polymorphic MHC class I molecules. Although certain members of the NKR-P1 family are expressed by all NK cells, we have identified a novel inhibitory rat NKR-P1 molecule termed NKR-P1C that is selectively expressed by a Ly49-negative NK subset with unique functional characteristics. NKR-P1C(+) NK cells efficiently lyse certain tumor target cells, secrete cytokines upon stimulation, and functionally recognize a nonpolymorphic ligand on Con A-activated lymphoblasts. However, they specifically fail to kill MHC-mismatched lymphoblast target cells. The NKR-P1C(+) NK cell subset also appears earlier during development and shows a tissue distribution distinct from its complementary Ly49s3(+) subset, which expresses a wide range of Ly49 receptors. These data suggest the existence of two major, functionally distinct populations of rat NK cells possessing very different killer cell lectin-like receptor repertoires.

Bioactive pectic polysaccharides from Glinus oppositifolius (L.) Aug. DC., a Malian medicinal plant, isolation and partial characterization

Glinus oppositifolius (L.) Aug. DC. (Aizoaceae) is a Malian medicinal plant used against various types of illnesses related to the immune response, like joint pains, inflammations, fever, malaria and wounds. Two pectin type polysaccharides, GOA1 and GOA2, being isolated from a 50 degrees C water extract from the aerial parts of Glinus oppositifolius were investigated for their activity towards the complement system and different leukocyte subsets because of the assumed effects on conditions related to the immune system. The polysaccharide polymer in GOA1 was shown to contain considerable amounts of the neutral sugars arabinose (26.4 mol%) and galactose (42.9 mol%), and methylation analysis indicated the presence of arabinogalactans type I (AG-I) and type II (AG-II). GOA2 was rich in galacturonic acid (68.3 mol%), along with rhamnose, arabinose and galactose. Structural studies indicated that rhamnose and galacturonic acid might constitute a rhamnogalacturonan backbone, often found in pectic substances, with side chains consisting of arabinose and galactose. Both GOA1 and GOA2 were shown to exhibit potent dose-dependent complement fixating activities, and induced chemotaxis of macrophages, T cells and NK cells.

Expression of regulator of G protein signalling proteins in natural killer cells, and their modulation by Ly49A and Ly49D

The small GTPase accelerators regulator of G protein signalling (RGS) proteins are important regulators of proximal signalling from G protein coupled receptors. Although natural killer (NK) cells express a number of G-protein coupled receptors, expression of RGS proteins has not been investigated. We analysed the expression of RGS proteins in rat NK cells, and detected mRNA for RGS1, RGS2, RGS5, RGS8, RGS16, and RGS18. Interestingly, when we included a panel of different leucocyte subsets, we found that RGS8 was selectively expressed by NK cells. NK cells are under control of both activating and inhibitory receptors and, utilizing a xenogeneic system where the mouse activating Ly49D or inhibitory Ly49A receptors were transfected into the rat RNK-16 cell line, the potential regulation of RGS proteins by single NK cell receptors was studied. We found that ligation of Ly49D led to a rapid and transient increase in message for RGS2, while Ly49A ligation up-regulated RGS2, RGS16, and RGS18 mRNA. Both receptors also induced a prolonged increase in RGS2 endogenous protein levels. These findings suggest that RGS proteins may be influenced by or involved in NK cell receptor events, suggesting a crosstalk between G-protein coupled receptors and NK cell receptors.

Two structurally related rat Ly49 receptors with opposing functions (Ly49 stimulatory receptor 5 and Ly49 inhibitory receptor 5) recognize nonclassical MHC class Ib-encoded target ligands

The Ly49 family of lectin-like receptors in rodents includes both stimulatory and inhibitory members. Although NK alloreactivity in mice is regulated primarily by inhibitory Ly49 receptors, in rats activating Ly49 receptors are equally important. Previous studies have suggested that activating rat Ly49 receptors are triggered by polymorphic ligands encoded within the nonclassical class Ib region of the rat MHC, RT1-CE/N/M, while inhibitory Ly49 receptors bind to widely expressed classical class Ia molecules encoded from the RT1-A region. To further investigate rat Ly49-mediated regulation of NK alloreactivity, we report in this study the identification and characterization of two novel paired Ly49 receptors that we have termed Ly49 inhibitory receptor 5 (Ly49i5) and Ly49 stimulatory receptor 5 (Ly49s5). Using a new mAb (mAb Fly5), we showed that Ly49i5 is an inhibitory receptor that recognizes ligands encoded within the class Ib region of the u and l haplotypes, while the structurally related Ly49s5 is an activating receptor that recognizes class Ib ligands of the u haplotype. Ly49s5 is functionally expressed in the high NK-alloresponder PVG strain, but not in the low alloresponder BN strain, in which it is a pseudogene. Ly49s5 is hence not responsible for the striking anti-u NK alloresponse previously described in BN rats (haplotype n), which results from repeated alloimmunizations with u haplotype cells. The present studies support the notion of a complex regulation of rat NK alloreactivity by activating and inhibitory Ly49 members, which may be highly homologous in the extracellular region and bind similar class Ib-encoded target ligands.

Phenotype and natural killer cell sensitivity of a radiation-induced acute T-cell leukaemia (Roser leukaemia) in PVG rats

A radiation-induced T-cell leukaemia [Roser leukaemia (RL)] in the rat was conditioned for growth in vitro by repeated in vivo-in vitro passages. This in vitro cell line, termed RL-T, maintained its leukaemia-inducing property when transferred to syngeneic PVG rats. It expresses several T-cell markers and the T-cell alpha/beta receptor-CD3 complex. RL-T, furthermore, expresses major histocompatibility complex (MHC) I antigens, both classical (RT1.A) and nonclassical (RT1.C), which makes it susceptible to killing by alloreactive natural killer cells in vitro.

cDNA cloning of a rat orthologue of SH2D2A encoding T-cell-specific adaptor protein (TSAd): expression in T and NK cells

The T-cell-specific adapter protein (TSAd), encoded by the SH2D2A gene, has been implicated in modulation of proximal signaling events as well as in transcriptional regulation in human T cells. We have isolated its rat homologue ( rSH2D2A) from an NK cell cDNA library and mapped the corresponding gene to chromosome 2 with a hamster-rat radiation hybrid cell panel. rSH2D2A encodes a 376 amino acid protein (rTSAd) which shows greater homology to mouse than human TSAd. In rats, rTSAd was specifically expressed by NK cells and T cells but not by other leukocytes tested. Similarly, in humans we observed abundant transcripts for TSAd in NK cells and T cells. The data suggest that TSAd may have a regulatory role in cellular activation of T and NK cells.

Activating and inhibitory Ly49 receptors modulate NK cell chemotaxis to CXC chemokine ligand (CXCL) 10 and CXCL12

NK cells can migrate into sites of inflammatory responses or malignancies in response to chemokines. Target killing by rodent NK cells is restricted by opposing signals from inhibitory and activating Ly49 receptors. The rat NK leukemic cell line RNK16 constitutively expresses functional receptors for the inflammatory chemokine CXC chemokine ligand (CXCL)10 (CXCR3) and the homeostatic chemokine CXCL12 (CXCR4). RNK-16 cells transfected with either the activating Ly49D receptor or the inhibitory Ly49A receptor were used to examine the effects of NK receptor ligation on CXCL10- and CXCL12-mediated chemotaxis. Ligation of Ly49A, either with Abs or its MHC class I ligand H2-D(d), led to a decrease in chemotactic responses to either CXCL10 or CXCL12. In contrast, Ly49D ligation with Abs or H2-D(d) led to an increase in migration toward CXCL10, but a decrease in chemotaxis toward CXCL12. Ly49-dependent effects on RNK-16 chemotaxis were not the result of surface modulation of CXCR3 or CXCR4 as demonstrated by flow cytometry. A mutation of the Src homology phosphatase-1 binding motif in Ly49A completely abrogated Ly49-dependent effects on both CXCL10 and CXCL12 chemotaxis, suggesting a role for Src homology phosphatase-1 in Ly49A/chemokine receptor cross-talk. Ly49D-transfected cells were pretreated with the Syk kinase inhibitor Piceatannol before ligation, which abrogated the previously observed changes in migration toward CXCL10 and CXCL12. Piceatannol also abrogated Ly49A-dependent inhibition of chemotaxis toward CXCL10, but not CXCL12. Collectively, these data suggest that Ly49 receptors can influence NK cell chemotaxis within sites of inflammation or tumor growth upon interaction with target cells.

Sphingosine 1 phosphate induces the chemotaxis of human natural killer cells. Role for heterotrimeric G proteins and phosphoinositide 3 kinases

We have examined the effect of sphingolipids on the chemotaxis of human natural killer (NK) cells. Messenger RNA for Edg-1, Edg-6 and Edg-8 but not Edg-3, are expressed in these cells. Sphingosine 1 phosphate (SPP), dihydro SPP (DHSPP) or the CC chemokine RANTES (CCL5), but not sphingosine induces the chemotaxis of these cells. Pertussis toxin inhibits the chemotaxis induced by these ligands. Permeabilization of NK cells with streptolysin O (SLO) and introduction of blocking antibodies to the heterotrimeric G proteins, showed that Galpha(i2), Galpha(s), Galpha(q/11) or Galpha(13) mediate the chemotaxis of SPP, whereas Galpha(i2), Galpha(o) or Galpha(q/11) mediate the chemotaxis of DHSPP. Galpha(i2), Galpha(o), Galpha(s), Galpha(q/11), Galpha(z) or Galpha(12 )mediates RANTES-induced NK cell chemotaxis. Further analysis showed that phosphoinositide 3 kinase (PI3K) inhibitors wortmannin and LY294002 inhibit NK cell chemotaxis induced by SPP, DHSPP or RANTES. Blocking antibody to PI3Kgamma inhibits the chemotaxis induced by the three ligands, whereas anti-PI3Kbeta was without effect. In contrast, SPP and DHSPP recruit PI3Kbeta isozyme into NK cell membranes, suggesting that although this isoform is not involved in chemotaxis, it is activated by these phospholipids.

Ly49i2 is an inhibitory rat natural killer cell receptor for an MHC class Ia molecule (RT1-A1c)

We have exploited strain-specific differences in the NK allorecognition repertoires to generate rat monoclonal antibodies against receptors involved in the control of allogeneic responses by rat NK cells. The monoclonal antibody STOK2 binds to a homodimeric glycoprotein that has been implicated as an inhibitory receptor for an MHC molecule in the PVG strain. In the present study, we haveidentified this glycoprotein as a novel rat Ly49 receptor (Ly49i2) containing an immunoreceptor tyrosine-based inhibitory motif. Ligation of the Ly49i2 receptor induces inhibitory signals, and Ly49i2 coprecipitates with the inhibitory tyrosine phosphatase SHP-1 in stably transfected RNK-16 cells. Moreover, it inhibits natural killing of lymphoblast targets and transfected fibroblast targets expressingthe classical MHC class Ia allele RT1-A1(c). Ly49i2, therefore, is an inhibitory receptor for specific MHC class Ia molecules, similar to inhibitory members of the mouse Ly49 family.

Characterization of a novel killer cell lectin-like receptor (KLRH1) expressed by alloreactive rat NK cells

NK cells have the ability to recognize and kill MHC-mismatched hemopoietic cells. In the present study, strain-specific differences in the rat NK allorecognition repertoire were exploited to generate Abs against receptors that may be involved in allogeneic responses. A mAb termed STOK9 was selected, and it reacted with subsets of NK cells and NKR-P1(+) T cells from certain rat strains possessing highly alloreactive NK cells. The STOK9(+) NK subset was broadly alloreactive and lysed Con A lymphoblast targets from a range of MHC-mismatched strains. The mAb STOK9 precipitated a 75-kDa dimeric glycoprotein from NK lysates. Expression cloning revealed that each monomer consisted of 231 aa with limited homology to other previously characterized killer cell lectin-like receptors (KLRs). This glycoprotein therefore constitutes a novel KLR branch, and it has been termed KLRH1. A gene in the central region of the natural killer gene complex on rat chromosome 4 encodes KLRH1. A mouse homolog appears to be present as deduced from analyses of genomic trace sequences. The function of KLRH1 is unknown, but it contains an immunoreceptor tyrosine-based inhibitory motif, suggesting an inhibitory function. The MHC haplotype of the host appears to influence KLRH1 expression, suggesting that it may function as an MHC-binding receptor on subsets of NK cells and T lymphocytes.

The athymic nude rat: an animal experimental model to reveal novel aspects of innate immune responses?

Athymic nude rats resemble nude mice in their lack of a normal thymus and functionally mature T cells. They have been useful in the study of mechanisms of tumor growth or graft rejection in immunocompromised hosts since they can accept major histocompatibility complex (MHC) mismatched organ allografts or xenografts for several months and because a number of tumor cell lines of human and rodent origin grow well in these rats. Injection of a few helper T (Th) cells from euthymic littermate rats partly restores the pool of mature T cells as well as full immunocompetence to reject organ allografts and has helped to reveal some of the cell interactions necessary for rejection to occur In contrast, immunologically naive athymic nude rats of certain strains, acutely reject allografts consisting of lymphocytes or bone marrow cells, which is due to the presence of alloreactive natural killer cells. These cells can recognize and kill MHC incompatible hematopoietic cells through the recognition of both mismatches within the classical (RT1.A) and nonclassical (RT1.C/E) MHC class I regions with a repertoire of inhibitory and activating killer lectin-like receptors (KLR) for MHC-I molecules, encoded by the Ly-49 portion of the rat natural killer cell gene complex (NKC). Some of these receptors have been identified and molecularly cloned and show similarities with NK receptors identified in the mouse. Other leukocytes in nude rats, such as dendritic cells, may also contribute to specific innate immune responses in the absence of mature T cells. Nude rats develop T-like cells expressing CD3 and T-cell receptor (TCR) with increasing age. Though their phenotype in peripheral lymphatic tissues resembles that of normal T cells, consisting mainly of CD4+ or CD8+ cells, they lack alloreactivity in vivo and their TCR repertoire is more of an oligoclonal nature. Their contribution to allograft rejection in T-cell-reconstituted rats is therefore questionable, and their role in innate immune response in these rats still enigmatic.

The effect of in vivo depletion of NKR-P1+ or CD8+ lymphocytes on the acute rejection of allogeneic lymphocytes (ALC) in the rat

We have depleted lymphocyte subsets in PVG and AO rats with MoAbs 3.2.3 (against NKR-P1 on NK and NK/T cells) and OX-8 (against CD8 on CTL and NK cells), and examined the effect on the killing of YAC-1 target cells in vitro and the effect on the acute rejection of small allogeneic lymphocytes in vivo (allogeneic lymphocyte cytotoxicity, ALC). While 3.2.3 treatment led to only a partial depletion of 3.2.3-positive cells in PVG rats, this treatment drastically reduced the number of NKR-P1+ cells in AO rats, abolished splenic NK activity against the NK-sensitive tumour target YAC-1, and markedly diminished the ALC response. Rats treated with OX-8 for 1 day showed a similar loss of NK cell function in vivo and in vitro. However, in rats treated with OX-8 for 3 days a 3.2.3+ and OX-8- population consisting of NK cells appeared, restoring ALC. The results demonstrate that NK cell responses can be greatly diminished after in vivo treatment with these MoAbs. Furthermore, they demonstrate that ALC is not necessarily linked to expression of the CD8 molecule.

Release from tonic inhibition of T cell activation through transient displacement of C-terminal Src kinase (Csk) from lipid rafts

In resting peripheral T cells, Csk is constitutively present in lipid rafts through an interaction with the Csk SH2-binding protein, PAG, also known as Cbp. Upon triggering of the T cell antigen receptor (TCR), PAG/Cbp is rapidly dephosphorylated leading to dissociation of Csk from lipid rafts. However, tyrosine phosphorylation of PAG/Cbp resumes after 3--5 min, at which time Csk reassociates with the rafts. Cells overexpressing a mutant Csk that lacks the catalytic domain, but displaces endogenous Csk from lipid rafts, have elevated basal levels of TCR-zeta-chain phosphorylation and spontaneous activation of an NFAT-AP1 reporter from the proximal interleukin-2 promoter as well as stronger and more sustained responses to TCR triggering than controls. We suggest that a transient release from Csk-mediated inhibition by displacement of Csk from lipid rafts is important for normal T cell activation.

Long-term donor chimerism after MHC (RT1) mismatched bone marrow transplantation in the rat: the role of host alloreactive NK cells

We have investigated the role of major histocompatibility complex (MHC) (RT1) disparities in the engraftment of bone marrow (BM) cells after whole body irradiation of rats. Mononuclear BM cells from PVG.RT7.2 (RT1c) rats were injected i.v. into sublethally (10Gy) whole body irradiated PVG (RT1c) rats and RT1 congenic and recombinant PVG rats. Repopulation of the BM, spleen, and blood with donor cells was assessed by FACS analysis of cells labelled with the fluorescein isothiocyanate (FITC)-labelled HIS41 monoclonal antibody (MoAb) against the RT7.2 marker. In RT1 matched (PVG.RT7.2 --> PVG) and RT1-mismatched combinations (PVG.RT7.2 --> PVG.1AV1), where radioresistant host natural killer (NK) cells could not recognize the BM inoculum as foreign, a donor chimerism close to 100% was observed after 6-8 weeks. However, in rat strain combinations where host NK cells could recognize an RT1 mismatch, almost no donor cells survived, and the rats were repopulated with leukocytes of host origin. In intra-MHC recombinant rat strains the element determining rejection or acceptance of the allograft mapped to the RT1-B/D-C/E/M region in PVG.R8 and PVG.R23 rats, in accordance with the patterns of NK alloreactivity in these strain combinations. NK cells may therefore be a primary obstacle to successful allogeneic BM engraftment in this model.

Rat natural killer cell receptor systems and recognition of MHC class I molecules

Rat natural killer (NK) cells recognize MHC-I molecules encoded by both the classical (RT1-A) and non-classical (RT1-C/E/M) MHC class I (MHC-I) regions. We have identified a receptor, the STOK2 antigen, which belongs to the Ly-49 family of killer cell lectin-like receptors, and we have localized the gene encoding it to the rat natural killer cell gene complex. We have also shown that it inhibits NK cytotoxicity when recognizing its cognate MHC-I ligand RT1-A1c on a target cell. This is the first inhibitory Ly-49-MHC-I interaction identified in the rat and highlights the great similarity between rat and mouse Ly-49 receptors and their MHC ligands. However, the mode of rat NK-cell recognition of target cells indicates that positive recognition of allo-MHC determinants, especially those encoded by the RT1-C/E/M region, is a prevalent feature. NK cells recruited to the peritoneum as a consequence of alloimmunization display positive recognition of allodeterminants. In one case, NK cells activated in this way have been shown to be specific for the immunizing, non-classical class I molecule RT1-Eu. These findings show that allospecific NK cells sometimes show features reminiscent of the adaptive immune response.

C1qRp elicits a Ca++ response in rat NK cells but does not influence NK-mediated cytotoxicity

The cell surface receptor C1qRp (receptor for C1q, regulating phagocytosis) present on macrophages and neutrophils, is presumed to stimulate phagocytosis in these cells. However, C1qRp is also present on natural killer (NK) cells, and in these cells its physiological function is not currently known. We have investigated putative functions of this cell surface molecule in rat NK cells with the aid of two novel monoclonal antibodies (MoAb) LOV3 and LOV8 against rat C1qRp. NK cells are known to be potent cytotoxic effector cells, both through specific recognition of ligands on a target cell and killing of antibody-coated target cells (antibody-dependent cellular cytotoxicity, ADCC). NK cells prestimulated with MoAbs LOV3 or LOV8 did not exhibit altered ADCC. Furthermore, the addition of MoAb LOV3 or LOV8 to cytotoxic cultures of NK cells and Fc-receptor positive tumour cells did not affect killing in a redirected killing assay, indicating that the receptor did not influence NK cytotoxicity. However, this is the first paper to show that an intracellular Ca++-response is induced in rat NK cells upon stimulation of C1qRp with LOV3 and LOV8. The response induced by the antibodies was only minimally reduced in the presence of EGTA, indicating that most of the response is owing to the Ca++ mobilization from intracellular calcium stores.

A soluble LAT deletion mutant inhibits T-cell activation: reduced recruitment of signalling molecules to glycolipid-enriched microdomains

The type III transmembrane adaptor protein linker for activation of T cells (LAT) is essential for membrane recruitment of signalling molecules following TCR activation. Here we show that although LAT deleted in the transmembrane domain is completely soluble, it can be tyrosine phosphorylated after anti-CD3 stimulation or pervanadate treatment. Overexpression of this deletion mutant in transiently transfected Jurkat TAg cells inhibits transcriptional activation of nuclear factor of activated T cells (NF-AT)/AP-1 reporter construct in a concentration-dependent manner. Furthermore, by selection of transiently transfected cells, a clear reduction of TCR-induced CD69 expression was observed in cells expressing the mutant. These dominant negative effects seemed to be dependent both on the ability of the membrane deletion mutant to reduce phosphorylation of endogenous LAT and to reduce interaction of endogenous LAT with PLC-gamma1 and Grb2. Consistent with this, the redistribution of PLC-gamma1 and Grb2 to glycolipid-enriched microdomains, called lipid rafts, after stimulation was inhibited when the soluble form of LAT was overexpressed. We suggest that the dominant negative effect is caused by the ability of the mutant to sequester signalling molecules in cytosol and thereby inhibit redistribution of signalling molecules to lipid rafts upon T-cell activation.

Characterization and molecular cloning of rat C1qRp, a receptor on NK cells

Here we report the generation of monoclonal antibodies (mAb) LOV3 and LOV8 to a 110-130-kDa membrane glycoprotein expressed by rat NK cells. This NK surface molecule was identified by eucaryotic expression cloning as the structural orthologue of the phagocytosis-stimulating receptor for complement factor C1q and mannose-binding lectin on human macrophages, C1qRp. Rat C1qRp is a monomeric type I integral membrane protein consisting of 643 amino acids with an N-terminal lectin-like domain, five epidermal growth factor-like domains, a transmembrane domain and a 45-residue cytoplasmic domain. It is encoded by a single gene on rat chromosome 3q41-q42 and is 67% and 87.5% identical at the amino acid level to human and mouse C1qRp, respectively. Rat C1qRp is expressed by resting and by activated NK cells, on subpopulations of NKR-P1(+) T cells (NK/T cells), dendritic cells, macrophages and granulocytes, but not by B cells or NKR-P1(-) T cells. Expression of this innate immune receptor is therefore not restricted to hematopoietic cells of the myeloid lineage, but is also expressed on subsets of cells of lymphoid origin. The mAb did not affect the cytotoxic function of NK cells, and C1qRp on NK cells may have functions not related to NK killing.

[Natural killer cells--important in the immunologic defense]

Natural Killer (NK) cells are a group of lymphocytes with a characteristic morphology and function. They are abundant in blood, spleen, liver and lungs. They are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way they discriminate between self and nonself. This latter NK function can partly be explained by an array of recently characterised NK receptors that can recognise and accurately discriminate between normal and altered MHC class I determinants. The basis for this discrimination is different from that of T cells and is discussed in this article. The role of NK cells in antimicrobial defense is well demonstrated, particularly that against viruses belonging to the herpesvirus group. A case report of a patient with a selective defect in NK cells and with recurrent viral infections is described. The role of NK cells in defense against malignant cells is more circumstantial, but NK cells do possess receptors which recognise tumour cells and kill them efficiently in vitro. A receptor which can recognise determinants unique for cancer cells has recently been described.

Natural Killing of Xenogeneic Cells Mediated by the Mouse Ly-49D Receptor

NK lymphocytes lyse certain xenogeneic cells without prior sensitization. The receptors by which NK cells recognize xenogeneic targets are largely uncharacterized but have been postulated to possess broad specificity against ubiquitous target ligands. However, previous studies suggest that mouse NK cells recognize xenogeneic targets in a strain-specific manner, implicating finely tuned, complex receptor systems in NK xenorecognition. We speculated that mouse Ly-49D, an activating NK receptor for the MHC I ligand, H2-Dd, might display public specificities for xenogeneic target structures. To test this hypothesis, we examined the lysis of xenogeneic targets by mouse Ly-49D transfectants of the rat NK cell line RNK-16 (RNK.Ly-49D). Of the xenogeneic tumor targets tested, RNK.Ly-49D, but not untransfected RNK-16, preferentially lysed tumor cells derived from Chinese hamsters and lymphoblast targets from rats. Ly-49D-dependent recognition of Chinese hamster cells was independent of target N-linked glycosylation. Mouse Ly-49D also specifically stimulated the natural killing of lymphoblast targets derived from wild-type and MHC-congenic rats of the RT1lv1 and RT1l haplotypes, but not of the RT1c, RT1u, RT1av1, or RT1n haplotypes. These studies demonstrate that Ly-49D can specifically mediate cytotoxicity against xenogeneic cells, and they suggest that Ly-49D may recognize xenogeneic MHC-encoded ligands.

Natural killing of xenogeneic cells mediated by the mouse Ly-49D receptor

NK lymphocytes lyse certain xenogeneic cells without prior sensitization. The receptors by which NK cells recognize xenogeneic targets are largely uncharacterized but have been postulated to possess broad specificity against ubiquitous target ligands. However, previous studies suggest that mouse NK cells recognize xenogeneic targets in a strain-specific manner, implicating finely tuned, complex receptor systems in NK xenorecognition. We speculated that mouse Ly-49D, an activating NK receptor for the MHC I ligand, H2-Dd, might display public specificities for xenogeneic target structures. To test this hypothesis, we examined the lysis of xenogeneic targets by mouse Ly-49D transfectants of the rat NK cell line RNK-16 (RNK. Ly-49D). Of the xenogeneic tumor targets tested, RNK.Ly-49D, but not untransfected RNK-16, preferentially lysed tumor cells derived from Chinese hamsters and lymphoblast targets from rats. Ly-49D-dependent recognition of Chinese hamster cells was independent of target N-linked glycosylation. Mouse Ly-49D also specifically stimulated the natural killing of lymphoblast targets derived from wild-type and MHC-congenic rats of the RT1lv1 and RT1l haplotypes, but not of the RT1c, RT1u, RT1av1, or RT1n haplotypes. These studies demonstrate that Ly-49D can specifically mediate cytotoxicity against xenogeneic cells, and they suggest that Ly-49D may recognize xenogeneic MHC-encoded ligands.